The present invention relates to a method of treating a high hydrohead fibrous porous web material and, for example, to a method that increases the retentive water absorbency of the web. The retentive water acquision rate may also be increased. As used herein the term "absorbency" generally refers to the ability of a material to acquire a fluid and the acquision rate refers to the rate of such acquision. An example of a use where high absorbency and high acquision are desired would be wiper type materials. In addition to having the characteristics of high absorbency and high acquision rate, wipers desirably also should have the characteristics of high retentive absorbency and high retentive acquision rate. The term retentive acquision rate is used herein to designate comparison of the rate of acquision of a fluid by a material when the material is first used to acquire the fluid as compared to the second, third and fourth times the material is used to acquire the fluid. Improved retentive acquisition rate is evidenced by by smaller decreases in the rate of acquisition with multiple uses. Likewise, the term retentive absorbency is used to designate comparison of the amount of fluid acquired by a material when the material is first used to acquire the fluid as compared to the amount of fluid acquired when the material is used a second, third, fourth time to acquire the fluid. Improved retentive absorbency is evidenced by smaller decreases in the amount of fluid absorbed by the material with multiple uses. In other words, the ability of the material to reabsorb fluid after having, in our test, been exposed to fluid, wrung out and allowed to dry.
In the copending, concurrently filed application Ser. No. 07/608,103 of Bernard Cohen and Michael T. Morman entitled Low Hydrohead Fibrous Porous Web with Improved Retentive Wettability the inventors disclose that the application of corona discharge treatment to low hydrohead webs whose surface includes a surface active agent having a hydrophile-lipophile balance of 6 or greater results in a significant increase in the retentive wettability, as defined therein, of such webs. This application is hereby incorporated by reference. Low hydrohead webs of that type would generally be unsatisfactory for use as a wiper material due to their open pore structure which would greatly reduce the ability of the web to acquire fluids. Conversely, high hydrohead webs, as defined herein, would generally be undesirable for use in applications where rapid transmission of large amounts of fluid through the material is desired. This undesirability arises from the generally tight, closed pore structure of high hydrohead materials. Such pore structure would inhibit the passage of fluids therethrough in rapid fashion.
In the past, hydrophobic wipers have been subjected to treatment with surfactants to improve their characteristics. The wipers have been treated with surfactant by (1) passing the formed wiper through a bath containing the surfactant in either neat or solution form and drying the wiper as needed so that a given amount of the surfactant is deposited on the wiper, or (2) spraying a surfactant in either neat or solution form on the fibers as they are being formed or on the fibrous porous web and drying the wiper as needed so that a given amount of the surfactant is deposited on the wiper, or, (3) adding surfactant to a thermoplastic resin prior to extrusion and formation of the resin into a thermoplastic porous web material. In the later situation, under known process conditions, the added surfactant exudes or migrates to the surface of the fibres of the porous web material during or shortly after fiber formation. This phenomenon has been referred to as "blooming" the surfactant. It is believed that blooming results from the insolubility of the surfactant in the thermoplastic polymer as the polymer cools. See U.S. Pat. No. 4,535,020 to Thomas et al (hereafter Thomas et al 020) which demonstrates surfactant blooming in a diaper liner formed from a perforated film.
A wiper made from a hydrophobic material, such as a thermoplastic polymer, will not readily acquire or absorb spilled fluids because the surface tension of the fluid is greater than the critical surface energy of the hydrophobic material. Surface tension is the contractile surface force of a fluid where the fluid tries to assume a spherical form and to present the least possible surface area. It is usually measured in dynes per centimeter. Accordingly, because of its effect on the insulating fluids, surfactant has been previously applied to wipers. Application of a surfactant onto a wiper material may make a nonabsorbing wiper absorbent by at least two mechanisms: (1) Surfactants present on the wiper can dissolve into a fluid and lower the surface tension of the resulting solution to more equal the critical surface energy of the wiper material. Accordingly, when a surfactant coated wiper is used to wipe up a fluid such as water, the surfactant acts to lower the surface tension of the fluid and allow the fluid to be acquired at a faster rate and for a larger amount of fluid to be absorbed into the wiper. In this situation, a certain amount of the surfactant on the wiper is lost with each wiping and wringing and unacceptable acquision rate and absorbency occurs at some following wiping due to the lack of availability of surfactant to lower the surface tension of the fluid. (2) The surfactant can be coated onto the fibers making up the wiper, making the fiber surface of the wiper more hydrophilic, i.e., increase the apparent critical surface energy of the fibers. In this situation the wiper would have permanent absorbency if the surfactant did not dissolve in the fluid the wiper was used to pick up.
As any anyone will testify, it is an aggravating event when a disposable wiper fails in its appointed task of rapidly acquiring and absorbing a fluid spill.
Accordingly, it has been a goal of those in the art to provide a high hydrohead porous web wiper material which has an improved acquision rate and absorbency. This was the initial goal because, if the material cannot acquire and absorb fluid at all, the material cannot function as a wiper. Additionally, it has been a goal of those in the art to provide a high hydrohead porous web wiper material which has an improved retentive acquision rate and improved retentive absorbency. That is, when dried and wrung-out between wipings, the wiper has a significant increase in the number of times it can be used to absorb fluid. This goal is desirable not only from the standpoint of allowing a given disposable wiper to be used more times but also from an environmental standpoint in that fewer wipers will be disposed into the environment.
Corona discharge treatment of films is old in the art and it is known that corona discharge treatment of a polymer film in the presence of air entails substantial morphological and chemical modifications in the polymer film's surface region. See Catoire et al, "Physico-chemical modifications of superficial regions of low-density polyethylene (LDPE) film under corona discharge," Polymer, vol. 25, p. 766, et. seq, June, 1984.
Generally speaking, corona treatment has been utilized to either (1) improve the print fastness on the film, or (2) to perforate the film. For example, U.S. Pat. No. 4,283,291 to Lowther describes an apparatus for providing a corona discharge, and U.S. Pat. No. 3,880,966 to Zimmerman et al discloses a method of using a corona discharge to perforate a crystalline elastic polymer film and thus increase its permeability. U.S. Pat. No. 3,471,597 to Schirmer also discloses a method for perforating a film by corona discharge. U.S. Pat. No. 3,754,117 to Walter discloses an apparatus and method for corona discharge treatment for modifying the surface properties of thin layers of fibers which improve the adhesion of subsequently applied inks or paints or of subsequent bonding.
It also is possible to treat a diaper liner material with a corona discharge and then immediately dip the film in a surfactant solution. Because the corona effect on the material generally starts to immediately decay, it is important to get the corona treated material into the bath as quickly as possible. Such a method is discussed in Japanese KOKAI Patent Number SHO63[1988]-211375. This document discloses a method for producing a nonwoven fabric having a long lasting hydrophilicity. The method involves first treating a nonwoven fabric of synthetic fiber by a corona discharge and then coating the treated fabric with about 2-10 grams per square meter of fabric of surface active agent.
Of particular interest is the fact that Thomas et al 020 is directed to the utilization of corona discharge in conjunction with surfactant treated films to effect improved wettability, i.e. higher fluid transmission rates and therefore decreased run-off of fluid. In this regard Thomas et al 020 states that a perforated film which has been treated with surfactant and which is then corona discharge treated results in a film with very low, zero or near zero fluid run-off on the first run-off test. Thomas et al 020 reports that this effect is accomplished because the corona discharge treatment acts on the chemical additive, the surfactant, to provide the perforated film with a zero or near zero percent run off. Thomas et al 020 postulates that this effect is achieved due to the surfactant providing a greater polarizability to the film then the film would have without the surfactant being added. The corona discharge treatment provides additional polarizing effect and, in combination with the surfactant, provides improved wettability. Because Thomas et al 020 is directed toward use of the perforated film as a diaper liner, it does not appear to address the questions of acquision rate and absorbency. Acquision rate, as defined herein, usually does not apply to a film and diaper liners are generally designed to be permeable to fluids as opposed to absorbing them. Lastly, Thomas et al 020 does not appear to address retentive capabilities at all because the testing reported therein is directed to one-time exposure to fluid.
In view of the forgoing, and the discovery by Messers, Cohen and Morman that treating a low hydrohead porous web with a surface active agent having a hydrophile-lipophile balance of about 6 or greater followed by corona discharge treatment yielded significantly improved retentive wettability values for the material, we decided to determine if such treatment had advantageous effects on the retentive water acquision rate and retentive water absorbency of high hydrohead porous webs. If such was the case an improved wiper would result.